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Ichthyology 3
Field techniques lecture 4
Ageing and diet analysis
Methods for age determination:
1. Length-frequency analysis
2. Hard part analysis
3. Direct Observation of individuals
Fig
ure
7.
Cro
ss s
ection o
f oto
lith
from
a 5
51m
m F
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oensk
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(Chry
soble
phus
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th
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ith O
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and r
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red a
fter
2102 d
ays
at libert
y.
Magnif
ied s
ection t
aken u
nder
transm
itte
d U
V a
nd r
efl
ecte
d n
orm
al li
ght.
2
45
1
3OT
C
OT
C 2
0/0
2/1
99
6
678910
22/1
0/2
001
4 x
10 x
Fig
ure
7.
Cro
ss s
ection o
f oto
lith
from
a 5
51m
m F
L p
oensk
op
(Chry
soble
phus
nasu
tus)
th
at
was
tagged a
nd in
jecte
d w
ith O
TC
and r
eca
ptu
red a
fter
2102 d
ays
at libert
y.
Magnif
ied s
ection t
aken u
nder
transm
itte
d U
V a
nd r
efl
ecte
d n
orm
al li
ght.
2
45
1
3OT
C
OT
C 2
0/0
2/1
99
6
678910
22/1
0/2
001
2
45
1
3OT
C
OT
C 2
0/0
2/1
99
6
678910
22/1
0/2
001
4 x
10 x
A B
Length frequency analysis • THEORY: Follow the progression of lengths over time.
This was originally developed for tropical populations
where there were thought to be no discernable growth
checks on hard parts
• SUITABLE FOR: Fast growing fish with distinct
spawning season or for young individuals of
slower growing fish. Stable environments are
better.
• DATA REQUIREMENTS – Preferably monthly
but at minimum, bimonthly (every two months)
length frequency data. However, if you have a
very strong cohort coming through, the sampling
can be less frequent.
• SAMPLING BIAS - Must assume or demonstrate
that there is no gear bias? (i.e. gillnets and hook
and line are not suitable (WHY???), purse seine
and seine nets with small mesh size most
suitable)
Hard Part analysis
• Scales
• Vertebrae
• Spines
• Opercular bones
• Otoliths
Hard Part Analysis
• THEORY: Aging is based on the appearance of
marks on a part that correspond with temporal
units (years or days).
• It is thought that annual marks are related to
seasonal changes in growth, while daily
increments are formed by differential deposition
of calcium and protein over a 24 hour period
(Circadian rhythm)
• SUITABLE FOR: All fishes, although it was
initially assumed that tropical fishes did not
deposit marks.
Bone (Vertabrae, spines, bones) • Spines
• Monkfish the first modified spine is used for ageing.
• Opercular bones used in Tilapia.
• Vertebrae use for sharks which do not have useable otoliths.
• POTENTIAL BIAS WHEN USING BONE: Resorbtion and asymptotic growth present problems.
Age (years)
0 2 4 6 8
Tota
l le
ngth
(m
m)
0
50
100
150
200
250
L50
This study (L = 211 mm TL; K = 0.24;to = -1.36)
Duponchelle et al. 2000 (L = 192 mm SL; K = 0.64; to = 0)
Allison et al. 1995 (L = 196 mmTL; K = 0.84; to= -0.28)
Otoliths • Paired calcified structures used for balance and/or
hearing.
• Annuli observed by Reibisch (1899) & daily growth increments by Pannella (1971).
• Otolith is acellular & metabolically inert.
• Otolith grows from before hatching to death.
• Currently the most popular hard part for ageing teleost fishes.
Whole otoliths
Problems with reading whole otoliths • Sometimes too thick.
• Stacking of growth zones later in life.
• OK for young and fast growing fish.
• Shown to significantly underestimate the
age of many long lived species
Sagittal
Transverse
Longitudunal SE
CT
ION
S
Reflected light Transmitted light
Reflected light
IMPORTANCE of Validation • Oxytetracycline
Validation: Marginal Zone Analysis
Percent
Month
Jan
Feb
Mar
Apr
May
Jun
Jul Aug
Sep
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
Jun
Jul Aug
Sep
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
Jun
Jul Aug
Sep
Oct
Nov
Dec
0
20
40
60
80
100
(a) (b) (c)
• DATA REQUIREMENTS – Depends on the validation
method (see Campana 2001). If using a MIA or MZA,
monthly sampling (minimum n = 30) required.
• If you have another validation type (eg. Chemical
marking), a minimum of 120 individuals of a wide size
range required (Larger species minimum requirement is
higher because they have more size classes). If there
are differences in growth rates between sexes, more fish
(200 or more) are required.
• SAMPLING BIAS – Gear type will not influence the
results directly, but you must try to get a sample with a
length frequency representative of the population under
study. This often means sampling in more than one
habitat with multiple gears
Direct Observations of Individuals
• THEORY: Fish are assumed to grow as they do in
natural environments. Their growth rate can be
monitored over time.
• SUITABLE FOR: Any species, but deep water species
not possible.
• Although this is accepted to be the most direct and
accurate of methods for quantifying the fish age, the are
biases associated with this method.
• DATA REQUIREMENTS – Obviously it is easy
to recapture stocked fish, but it is critical to
recapture and measure a sample of these fish
(for a length frequency histogram) at regular
intervals (at least twice a year). The experiment
should continue until the end of the lifespan of
the fish.
• SAMPLING BIAS – If spawning occurs in the
ponds, the results will be biased, especially later
on in the experiment. The ponds are not
necessarily always representative of the natural
environment.
Stomach content analysis
Why study gut contents…
• To assess the species nutritional standing in the context of the fish community. – Seasonal variation
– Comparison between sub-groups of same population.
– Feeding intensity throughout the day.
• Estimation of total amount of food consumed by a population. – Calculation of daily ration or energy budget based on field or
laboratory observations.
• Aquaculture – Larval rearing, optimal diets, daily rations, feeding regimes.
No Kill - Gastric Lavage
How do you do it?
Removing and storage of stomach contents
• Stomach contents or the whole stomach.
• Bind stomachs or guts.
• If no stomach take only the contents from a pre-determined section of the foregut.
• Contents should be removed from the fish as soon after capture as possible.
• 10% formalin or frozen.
• Label each stomach.
Which gear should you use?
PASSIVE GEAR - Longlines, gill nets & traps/fykes
• Fish continue digesting while they are trapped/hooked/gilled
(known as post capture digestion) so regular (hourly) retrieval
is essential.
• Worse, they may also regurgitate stomach contents.
• You are also fishing for actively feeding/moving fish. This
may affect results.
• Predatory fish in traps may actually eat other fish in the trap
and this obviously affects results.
ACTIVE GEAR- Seines/cast nets/ electro
fishing/rotenone (active gear)
• Very good but watch out for small fish being
pushed down the gullets of large fishes during
harvest.
• Regurgitation can also be a problem with all
techniques.
• With rotenone, large fish often feed on dying
smaller fish prior to succumbing to the poison.
– Hook and line
• May be biased as actively feeding fish are
selected.
• Bait type may pollute samples
• Where one is fishing may also affect results e.g. if
you are bottom fishing for kingfish then you are
likely to catch those fish that are feeding near the
bottom on crustaceans and miss those actively
surface feeding on baitfish elsewhere.
Best practices
• Fish should be sampled with gear that allows for quick removal of the fish from the environment and that do not bias the stomach content samples.
• Remove stomachs and preserve contents quickly.
• Try to sample populations after feeding events
• Requires prior sampling to determine peak feeding times. – If you are trying to determine peak feeding, then you
should compensate for digestion of stomach contents.
How do you describe
a fish’s diet?
• Occurrence
• Numerical
• Volumetric
• Gravimetric
• Subjective
• IRI= (%N + %V) x %F
Percent mass, percent numbers and percent frequency of occurrence of major
prey categories in the stomachs of Clarias gariepinus from (A) Katriver
reservoir; F = fish prey, P = plant material, I = insects, Z = zooplankton, O =
other.
Potts et al. 2008
%N %M %F IRI
Fish 0.02 52.81 53.49 2825.54
Plants 1.12 7.80 58.14 518.97
Insects 0.41 10.48 90.70 987.71
Zooplankton 98.37 1.25 25.58 2548.27
Other 0.11 22.96 32.56 751.31
Best practice.
• Use more than one method to describe
diet.
• Split fish into size classes when describing
the diet.
• Use fish size weight in gravimetric studies.
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